1. Field of the Invention
The present invention relates to an internal combustion engine ignition device able to suppress misfires etc. of an internal combustion engine at abnormal times and an igniter for the same.
2. Description of the Related Art
In the case of a spark ignition internal combustion engine fueled by gasoline, alcohol, etc. (hereinafter simply referred to as an xe2x80x9cenginexe2x80x9d), a secondary coil of an ignition coil is made to generate a high voltage and that high voltage is supplied to a spark plug so as to cause spark discharge across the gap of the spark plug. Due to this spark discharge, the compressed air-fuel mixture in the combustion chamber is ignited and burned. At this time, the discharge timing of the spark plug, that is, the ignition timing, largely governs the performance of the engine, so is precisely controlled in accordance with the rotational speed of the engine.
However, for example, when an abnormality etc. occurs in the electronic control unit (ECU) controlling the ignition timing and the ignition signal continues for a long time (for example, several seconds), the precise control of the ignition timing becomes impossible. Therefore, the air-fuel mixture of the engine can detonate due to a misfire and damage the engine etc. Even if such detonation of the air-fuel mixture does not occur, if the ignition signal continues for a long period of time, the primary coil of the ignition coil or the drive device (igniter) of the primary coil will be overheated by the large current running through it for the long period. Such overheating can become a cause of damage to the equipment or thermal runaway. Therefore, measures against such abnormal conditions have been disclosed for example in Japanese Unexamined Patent Publication (Kokai) No. 8-28415 and Japanese Unexamined Patent Publication (Kokai) No. 2002-4991.
In Japanese Unexamined Patent Publication (Kokai) No. 8-28415, at the time of an abnormality where the ignition signal continues for a long time, the drive signal of the switching device (power transistor) for controlling the primary current is dropped to the ground. Due to this, the primary current is cut and heating of the switching device etc. is suppressed. If this is done, however, in the end the primary current is rapidly cut and as a result a high voltage may arise at the secondary coil and spark discharge may occur at the spark plug. Therefore, it is not possible to reliably suppress misfires of the engine.
In the case of Japanese Unexamined Patent Publication (Kokai) No. 2002-4991, to make up for the defect of Japanese Unexamined Patent Publication (Kokai) No. 8-28415, at the time of detection of an abnormality, an insulated gate bipolar transistor (IGBT), a type of power transistor, is turned off at a low speed to prevent the occurrence of a high voltage at the secondary coil. Due to this, spark discharge of the spark plug is suppressed and misfire of the engine is reliably prevented. This low speed turnoff of the IGBT is achieved by slowly reducing the reference voltage of a comparator making up part of a current limiting circuit of the primary current. In the case of Japanese Unexamined Patent Publication (Kokai) No. 2002-4991, however, slow discharge of a capacitor is utilized at the time of reducing the reference voltage. Therefore, a capacitor is an essential device. However, with increasing thinness, smaller size, and lower cost being demanded, separate provision of a bulky capacitor is not preferable. Further, it is difficult to form such a capacitor on one chip. Further, along with the drop in the primary current control value, oscillation of the primary current value is liably to occur. It is difficult to prevent this oscillation.
An object of the present invention is to provide an internal combustion engine ignition device able to suppress misfires of an engine, overheating of an igniter etc., etc. even at an abnormal time at which an ignition signal continues for a long period etc. and able to achieve greater compactness and lower cost, and an igniter for the same.
To attain the above object, the present invention provides an internal combustion engine ignition device able to suppress misfires etc. at abnormal times and achieving smaller size and lower cost, provided with a power transistor (switching device SW), a current limiting circuit (1) for limiting a primary current running through the IGBT to within a predetermined value, a fast gate voltage drop circuit (2) for making the gate voltage of the IGBT fast drop to an extent where spark discharge occurs at the spark plug (P), an abnormality detecting circuit (6) for detecting an abnormal state of an igniter (I) or electronic control unit (9) and outputting an abnormality detection signal, and a slow gate voltage drop circuit (3) having a gate voltage feed cutoff circuit (42) for cutting off the feed of the gate voltage at an abnormal time and a discharge circuit (41) for discharging the gate capacitor charge of the IGBT to make the gate voltage slowly drop to an extent where no spark discharge will occur at the spark plug.